1. Field of the Invention
The present invention relates to a process using nitrogen ion implantation to increase the index of refraction near the surface of an amorphous silicon dioxide or glass substrate. The process is particularly suited for fabricating integrated optical waveguides and other integrated optical devices.
2. Description of the Prior Art
The field of integrated optics deals with the miniaturization and integration of optical and electro-optical devices using technologies derived from the field of microelectronics. As compared with larger discrete optics, integrated optics allow devices to be constructed more cheaply and much more compactly, and it facilitates interfacing between optical and electronic components.
Integrated optical devices are fabricated by modifying the optical propagation characteristics of selected regions of an optically transmissive substrate. The basic building block of an integrated optical device is an optical waveguide whose function in integrated optics is somewhat analagous to that of a conductor in microelectronics. A waveguide is fabricated in a substrate by forming a channel having a higher index of refraction than the surrounding regions. Light propagating along the channel will be guided and confined within the channel because of total internal reflection at the channel boundaries.
Amorphous silicon dioxide, in the form of silica or fused quartz, is widely used as a substrate for fabricating integrated optical waveguides because it is highly transparent at visible and infrared wavelengths; i.e., it offers low attenuation to visible and infrared light. The following references illustrate the chronological development of the art of optical waveguide fabrication in silicon dioxide substrates.
In 1972, Standley, Gibson, and Rodgers reported in "Properties of Ion-Bombarded Fused Quartz for Integrated Optics," Applied Optics, vol. 11, pages 1313-1316, June 1972, that the index of refraction of fused quartz could be increased by ion bombardment. They suggested fabricating low-loss optical waveguides using ion bombardment to increase the index of refraction of a channel in a quartz substrate. Standley et al. investigated the changes in the index of refraction of fused quartz due to implantation by bombardment with ions of He, Li, C, P, Xe, Tl, and Bi. They deduced that the desired changes in the refractive index, and the concomitant increase in the undesired propagation losses in the quartz, were primarily due to structural disorder produced by the bombardment rather than by chemical doping effects produced by the implanted particles. Thus, Standley, et al. did not disclose a means for increasing the refractive index of a substrate without a substantial increase in propagation losses.
Standley et al. concluded that most of the increase in refractive index achieved by the bombardment could be removed by annealing, which annealing also significantly reduced the propagation losses. Although their data indicates that a small residual change in the index of refraction due to bombardment by lithium ions remained after annealing, they apparently attached little significance to this residual effect and attributed it to "partial" annealing.
Standley et al. did not investigate the implantation of nitrogen ions. Also, they did not suggest that the changes caused by ion bombardment included significant chemical changes that might remain after annealing.
In 1976, in "Refractive Index Profiles Induced by Ion Implantation into Silica," J. Phys. D: Applied Physics, Vol. 9, 1976, pp. 1343-1354, printed in Great Britain, Webb and Townsend reported increases in the refractive index of silica (silicon dioxide) due to ion implantation by a number of different ions, namely H+, He+, Li+, B+, Na+, Ar+, Bi+, N+. They reported an increase in the refractive index of up to six percent for nitrogen and a one to two percent change in the index of the refraction for the other ions. Webb and Townsend suggested that the greater change in the index of refraction for nitrogen implantation might be due to a chemical process rather than to the compaction process which was thought to have caused the increases in the index when the other ions were used. Although Webb and Townsend attributed the greater increase in refractive index achieved with nitrogen ion bombardment as being due to a chemical change, they made no suggestion that the propagation losses in the substrate could be reduced by annealing, nor did they suggest that a major portion of the change in the index of refraction due to nitrogen ion implantation would remain after annealing.
Kersten, in U.S. Pat. No. 4,145,457, issued in 1979, disclosed a method for fabricating optical waveguides and directional couplers in quartz by implanting any one of a number of different ions, of which nitrogen was an example. Like Webb and Townsend, however, Kersten did not suggest the use of an annealing process to reduce the propagation loss in the waveguides, nor did he disclose that the nitrogen ions would produce a higher residual change in the index of refraction after annealing than the other ions considered.
Hubler et al disclosed in U.S. Pat. No. 4,262,056, issued in 1981, the use of nitrogen ions for bombarding a substrate to change the index of refraction for the purpose of fabricating a multilayer optical interference filter. However, the substrate material disclosed by Hubler, however, was silicon (Si) rather than silicon dioxide (SiO.sub.2). As will be illustrated below, the chemical properties of silicon and silicon dioxide are quite different, so that processes in one environment cannot be expected to apply to the other.
In Hubler et al., the implantation was performed at a high temperature of 600.degree. C. to 1000.degree. C. in order to cause the implanted ions to form silicon nitride (Si.sub.3 N.sub.4), and the substrate was maintained at this high temperature for a period of three to five hours during the implantation. Hubler et al. indicated that the high temperature removed the radiation damage to the crystallinity of the single-crystal layer of silicon overlying the implanted layer of silicon nitride, so that the optical properties of the overlying layer were unchanged from those of pure crystal and silicon. However, Hubler et al. did not discuss propagation losses in the implanted region or whether the high temperature affected such losses.
Also, Hubler et al. discloses that the nitrogen ion implantation of silicon substrates decreases the index of refraction in the implanted region, rather than increasing it as in the case of silicon dioxide substrates. This makes Hubler's process ill-suited to the fabrication of optical waveguides which require a channel having a higher refractive index than the surrounding substrate.
If Hubler's disclosed heating temperatures and times were applied to a silicon dioxide substrate rather than Hubler's disclosed silicon substrate, the heat would cause most of the implanted nitrogen ions to diffuse away from the implanted region, thus destroying the intended increase of the refractive index in that region.